Protestor Boss Bastain of St. Louis locks arms with others as they confront Missouri State Highway Patrol troopers in front of the Ferguson police station on Aug. 11. Marchers are entering a third day of protests against Sunday's police shooting of Michael Brown.Robert Cohen / St. Louis Post-Dispatch via AP

Police in Ferguson, Missouri, did not file an "incident report" on the fatal shooting of 19-year-old Michael Brown because they turned the case over to St. Louis County police almost immediately, the county prosecutor's office tells NBC News.

Critics and news media outlets have questioned why Ferguson police released an incident report from a robbery in which Brown was a suspect, as well as security video showing the robbery, but not the report on the shooting of the unarmed 18-year-old a short time later by Officer Darren Wilson.

The reason, according to the office of St. Louis County Prosecuting Attorney Robert P. McCulloch, is that it doesn't exist.

The St. Louis County police department presumably did file an incident report, but any such documents will not be made public until a grand jury investigating the officer-involved shooting concludes its investigation, according to officials from the office who briefed NBC News on the case.

The grand jury reviewing the facts in the case is impaneled until mid-September, but could continue to deliberate beyond its term, in which case their sole focus would be on the shooting of Brown. At the conclusion of its investigation, the grand jury will decide whether to indict Wilson in connection with the shooting.

The St. Louis County prosecutor's office stressed that it is cooperating with the concurrent federal investigation of Brown's death and is sharing information with FBI agents who are looking into whether his civil rights were violated.

The fatal shooting of Brown, an African American, on Aug. 9 by Wilson, who is white, triggered over a week of occasionally violent protest in the streets of Ferguson. The tensions appeared to ease somewhat on Wednesday, prompting Missouri Gov. Jay Nixon to begin withdrawing National Guard troops he ordered in to restore calm.